1. Field of the Invention
The present invention relates to a nonaqueous electrolyte for a lithium-ion secondary battery and a lithium-ion secondary battery.
2. Description of the Related Art
In the related art, as a positive electrode material (positive electrode active material) of a lithium-ion secondary battery, a laminar compound such as LiCoO2 or LiNi1/3Mn1/3Co1/3O2 or a spinel compound such as LiMn2O2 has been used. In recent years, a compound with an olivine structure represented by LiFePO4 has been focused on. It is known that a positive electrode material having an olivine structure has high thermal stability at a high temperature, and high security.
However, in the lithium-ion secondary battery using LiFePO4, there are problems in that charging/discharging voltage is low as about 3.5 V and energy density is low. Accordingly, as a phosphoric based positive electrode material which can realize high charging/discharging voltage, LiCoPO4, LiNiPO4, or the like has been proposed. However, even in a case of the lithium-ion secondary battery using the positive electrode materials, sufficient capacity is not obtained. Among the phosphoric based positive electrode material, as a compound which can realize 4 V charging/discharging voltage, vanadium phosphate having a structure of LiVOPO4 or Li3V2(PO4)3 or LiMnPO4 has been known.
In the positive electrode material of the related art, there has been a problem of swelling of lithium-ion secondary battery due to gas generation, in general. In addition, it has not been known that the gas generation occurs in the lithium-ion secondary battery using the phosphate compound, however, in practice, gas is generated, the lithium-ion secondary battery is swollen, and shape stability degradation problem occurs. In particularly, in a case of using a metal laminated exterior package, shape change is significant.
For example, Japanese Unexamined Patent Application Publication Nos. 2004-303527, 61-88466, and 5-6778 disclose about vanadium phosphate, however, do not disclose about the problem of gas generation. JP-A-61-88466 discloses about addition of vanadium salt in an electrolyte, however, does not disclose about the problem of gas generation.
JP-A-5-6778 discloses about suppression of gas generation of a battery, however, it was insufficient for countermeasure of gas generation in a case of using the positive electrode materials.